The present invention relates to a method for operating a multiple speed hydraulic motor. The present invention may be used, for example, in a compact construction vehicle such as a skid steer loader.
Compact construction vehicles typically include a prime mover, such as an internal combustion engine, which drives a hydraulic system. The hydraulic system includes hydraulic pumps that supply hydraulic fluid to hydraulic cylinders for raising and lowering lift arms, curling and dumping a bucket, and manipulating other auxiliary devices. The hydraulic system also includes one or more pumps that provide a flow of hydraulic fluid to hydraulic motors that rotate the wheels of the vehicle.
In some cases, hydraulic motors can be shifted between two or more operating speeds. For example, some hydraulic motors can be shifted between a low-speed high-torque (“LSHT”) mode while the vehicle is performing work and a high-speed low-torque (“HSLT”) mode while the vehicle is being driven between worksites. Such motors are shifted between LSHT and HSLT modes by changing the ratio of output shaft rotation to the flow rate of hydraulic fluid supplied by the hydraulic pump in the hydraulic system, instead of requiring the hydraulic system to provide a variable flow rate of hydraulic fluid.
Some types of multiple speed hydraulic motors are prone to premature failure if the motor is permitted to be shifted into a HSLT mode when the hydraulic fluid is too cold and viscous to adequately lubricate the internal friction surfaces of the drive motor. In addition, allowing a HSLT mode when the hydraulic fluid is too cold and viscous can produce a “thermal shock” condition causing pre-mature drive motor failure. This “thermal shock” can be caused by different rates of thermal expansions between components, pockets of high temp oil at certain location of a components or differences in temperature rise rate between components.
Some known hydraulic systems drive the hydraulic motors through a closed hydraulic circuit. There is typically some intentional loss of hydraulic fluid from closed hydraulic circuits to promote cooling of the components, and such loss of fluid can result in cavitation within the circuit which may lead to performance issues or even damage the motor. To alleviate such fluid loss, many hydraulic systems include a charge pump that operates in an open circuit. The charge pump draws fluid out of a reservoir, pressurizes the fluid, and communicates with the closed circuit. As a result, the closed circuit is pressurized by the charge pump to a “charge pressure,” and pressurized hydraulic fluid from the charge pump instantly supplements the hydraulic fluid in the closed circuit to make up for fluid losses. Such charge pumps typically operate at constant speed and are always on, whether the motor is in LSHT or HSLT mode of operation.